1. Field of the Invention
The present invention generally relates to an optical pickup for optically recording and/or reproducing information with respect to a record medium, and an information recording apparatus and an information reproducing apparatus having such an optical pickup. More particularly, the present invention relates to an optical pickup for recording and/or reproducing the information with compensating birefringence (or double refraction) generated in a light beam and a reflection beam thereof due to an optical characteristic of a light-transmissible layer which is disposed on the record medium and which transmits the light beam, and an information recording apparatus and an information reproducing apparatus having such an optical pickup.
2. Description of the Related Art
A so-called optical disc, for which various developments are being performed recently in various fields, is a record medium allowing to record and reproduce a large volume of information
In case of recording and reproducing information with respect to the optical disc by using a light beam such as a laser light, a structure as described below is often employed.
Namely, a linearly polarized light beam is firstly generated from a laser light source. Then, the light beam is transmitted through a polarization beam splitter (i.e., an optical element for reflecting or transmitting the light beam depending upon the direction of the polarization), and is inputted to a xc2xc wavelength plate. Then, the light beam, which has been linearly polarized, is converted into a circularly polarized light beam by a phase difference adding function of the xc2xc wavelength plate. After that, this light beam is inputted to an objective lens and is collected onto the optical disc.
On the other hand, the reflection light, which is generated as the optical disc reflects the light beam, becomes a circularly polarized light beam whose turn is reverse to that before the reflection. Then, as this light beam is transmitted again through the xc2xc wavelength plate, it becomes a linearly polarized light beam whose polarization direction is different by 90 degrees from that on the approaching path and is inputted again to the polarization beam splitter. Then, this light beam is reflected by the polarization beam splitter, is guided toward a light detector side without returning to the original laser light source side, is received by the light detector and is converted into an electric signal.
After this, in case of recording the information onto the optical disc, a position on the optical disc at which the information is to be recorded is detected on the basis of the electric signal, or the intensity of the light beam for recording is controlled. On the other hand, in case of reproducing the information from the optical disc, the information is reproduced on the basis of the electric signal.
Here, assuming that birefringence is generated in the light beam, when the light beam is transmitted through a substrate or a protection layer constituting the optical disc, due to an irregularity of material of the substrate or protection layer, the polarization of the light beam on the returning path reflected from the optical disc does not become the circularly polarized light beam precisely reverse to that on the approaching path. The polarization condition of the light beam on the returning path may be various in correspondence with the generated amount of the birefringence.
Then, when the light beam in such a condition is inputted again to the polarization beam splitter, all of the light beam is not reflected, but some portion of the light beam is transmitted and returns to the laser light side. As a result, the light detection amount at the light detector is decreased. In other wards, depending upon the amount of the birefringence generated in the light beam at the optical disc, the light detection amount at the light detector may be less than the limit for reproduction.
However, for example, among the CDs (Compact Discs) presently on sale, there exist a CD by which the generated amount of the birefringence is extremely large, and a CD by which the generated amount of the birefringence is drastically changed depending upon the position in the optical disc. Thus, there is a problem that a signal reproduction for such a CD cannot be performed by the above mentioned conventional reproduction optical system.
At this time, even in case. of using the conventional reproduction optical system, it is possible to compensate the birefringence generated by the optical disc by rotating the xc2xc wavelength plate. However, in this case, a mechanical and complicated control structure is required, so that the structure of the optical pickup itself becomes complicated and the production cost is drastically increased, which is another problem.
It is therefore an object of the present invention to provide an optical pickup, which can compensate the influence of the birefringence effectively by an electrical control without a mechanical control, and an information recording apparatus and an information reproducing apparatus having such an optical pickup.
The above object of the present invention can be achieved by an optical pickup provided with: a light emitting device such as a laser diode for emitting a light beam onto a record medium such as an optical disc comprising a transparent layer such as a transparent substrate, which is transparent for the light beam and through which the light beam is transmitted; a light receiving device such as a detector for receiving a reflection light of the light beam from the record medium; a liquid crystal compensating device such as a liquid crystal panel for compensating birefringence, which is generated in the light beam when the light beam is transmitted through the transparent layer, on the basis of the reflection light received by the light receiving device, by controlling an orientation direction of liquid crystal molecules in a liquid crystal layer so as to give a canceling phase difference, which is to cancel the birefringence, to the light beam transmitted through the liquid crystal layer; a generating device such as an amplitude detection circuit for generating an indication signal indicative of the birefringence on the basis of the reflection light received by the light receiving device; and a controlling device such as a driver for controlling the orientation direction on the basis of the indication signal so that a first angle between a standard line, which is perpendicular to an optical axis of the light beam and is set in advance, and the orientation direction in a plane perpendicular to the optical axis is kept to be constant and that a second angle between the optical axis and the orientation direction in a plane including the optical axis is changed, to thereby generate the canceling phase difference in the light beam transmitted through the liquid crystal compensating device.
According to the optical pickup of the present invention, the canceling phase difference is generated in the light beam just by changing the first angle of the orientation direction of the liquid crystal molecules. That is to say, the canceling phase difference is generated without changing the direction of the linearly polarized plane of the light beam (i.e., without rotating the direction of the linearly polarized plane). Therefore, it is possible to electrically compensate the birefringence generated in the light beam and the reflection light thereof effectively.
Further, since only the first angle of the orientation direction is changed, it is not necessary to employ the liquid crystal composition adapted to change the second angle of the orientation direction, so that it is possible to simplify the adjustment of the liquid crystal when constituting the liquid crystal compensating device.
In one aspect of the optical pickup of the present invention, the light receiving device generates a light detection signal, and the generating device generates the indication signal such that the indication signal indicates a cancellation of the birefringence by the canceling phase difference when the amplitude of the light detection signal becomes the maximum and outputs the generated indication signal to the controlling device.
Therefore, the birefringence is compensated as the controlling device controls the liquid crystal compensating device so that the amplitude of the light detection signal becomes the maximum, it is possible to compensated the birefringence by means of a relatively simple construction.
In another aspect of the optical pickup of the present invention, the liquid crystal compensating device comprises: the liquid crystal layer; two orientation films, which is disposed to sandwich the liquid crystal layer and is transparent for the light beam, for orienting the liquid crystal molecules such that the first angle is kept to be constant as for all of the liquid crystal molecules; and a voltage applying device such as a transparent electrode, which is disposed to sandwich the liquid crystal layer and the orientation films and is transparent for the light beam, for applying a control voltage, which is to control the second angle, to the liquid crystal molecules.
According to this aspect, since the first angle is made constant by the orientation films and since the canceling phase difference is generated as the voltage applying device changes the second angle, it is possible to compensate the birefringence effectively.
In this aspect of the optical pickup, the orientation films may be formed such that the first angle is 45 degrees with respect to each linearly polarized plane of the light beam before the light beam is transmitted through the liquid crystal layer, and the controlling device controls the orientation direction so that the canceling phase difference is generated in the light beam by changing the second angle with the second angle as a center, which gives to the light beam a phase difference corresponding to xc2xc of a wavelength of the light beam.
Accordingly, since the first angle of the orientation direction of the liquid crystal molecules is constant at the 45 degrees with respect to each linearly polarized plane of the light beam and since the orientation direction is controlled so that the canceling phase difference is generated in the light beam by changing the second angle with the second angle as a center, which gives to the light beam a phase difference corresponding to xc2xc of a wavelength of the light beam, the liquid crystal compensating device functions as a so-called xc2xc wavelength plate with respect to the light beam. Thus, it is not necessary to equip the xc2xc wavelength plate separately.
Therefore, it is possible to compensate the birefringence in the light beam while simplifying the structure of the optical pickup.
The above object of the present invention can be also achieved by an information recording apparatus provided with (A) the above described optical pickup of the present invention (including its various aspects), (B) a detecting device such as a reproduction circuit and a CPU for detecting record control information, which is recorded in advance on the record medium, on the basis of the reflection light received by the light receiving device, and (C) a modulating device such as a modulation circuit for modulating record signal to be recorded onto the record medium on the basis of the detected record control information, to thereby generate a modulation signal which controls the light emitting device to emit the light beam having an intensity corresponding to the modulation signal.
According to the information recording apparatus of the present invention, since the birefringence is not included in the light beam when detecting the record control information, it is possible to detect the record control information more precisely and record the record information onto the record medium.
The above object of the present invention can be also achieved by an information reproducing apparatus provided with (A) the above described optical pickup of the present invention (including its various aspects), and (B) a reproducing device such as a reproduction circuit for reproducing record information, which is recorded in advance on the record medium, on the basis of the reflection light received by the light receiving device, to thereby output a reproduction signal to an external portion.
According to the information reproducing apparatus of the present invention, since the birefringence is not included in the reflection light, it is possible to reproduce the record information more precisely.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.